Typically, printed circuit boards that require protection from moisture, electric leakage and dust are coated with moisture proof insulative films, known as conformal coatings, such as, acrylic, polyurethane, silicone or epoxy synthetic resins dissolved in a volatile solvent. One conformal coating found satisfactory is available from Humiseal Division, Columbia Chase Corp., Woodside, N.Y., under the trademark "HumiSeal 1B31". When applied to clean circuit boards, an insulative resin film of uniform thickness and without pinholes, is formed as the solvent evaporates.
In the past, five principal methods have been used to apply coatings of moisture proof, insulative material to printed circuit boards. These included the following:
(a) The immersion method, in which packaged circuit boards are immersed in an immersion tank containing the moistureproof insulative material.
(b) The brush-coating method, in which the moisture proof insulative material is applied by a brush to the printed circuit board surface to be coated.
(c) The roller method, in which a sheep's wool roll impregnated with the moisture proof insulative material is rolled on the surface of the printed circuit board to provide the insulative coating thereon.
(d) The spray method, in which the moisture proof insulative material is applied to the printed circuit board by spraying techniques.
(e) The slit die method, in which the moisture proof insulative material is pressurized and extruded from a slit die to eject a film for coating the printed circuit board surface.
Each of the foregoing methods have certain advantages and disadvantages. For example, all methods except brush-coating and certain spraying techniques require masking for those parts and regions on the circuit board to be left uncoated. The masking operations, that is, mounting and removal of the masks, must be done manually, causing a bottleneck in the mass production process. Brush-coating, while not requiring masking, is labor intensive, and otherwise unsuitable for mass production.
Spraying is the most commonly used insulative coating method employed in mass production. Spraying can be categorized as either air spraying in which an air stream is impinged against the stream of liquid coating material after leaving the spray nozzle to atomize it, and airless spraying in which the coating material is dispensed in a low pressure, unatomized flat film pattern, as is disclosed in U.S. Pat. No. 4,880,663, assigned to Nordson Corp., which is hereby incorporated by reference in its entirety.
With air spraying of conformal material onto a circuit board, the air, at a pressure of generally about 30 to 40 pounds per square inch (psi), is projected in the form of opposed air jets against a stream of liquid conformal coating material which is dispensed at a pressure of generally about 20 to 100 psi from a spray nozzle. The required pressures are dependent upon a number of factors including the viscosity of the conformal coating material, the temperature of the conformal coating material and the size of the nozzle. With air spraying, a high flow rate results in thick coatings, which are undesirable, and relatively uncontrolled spray patterns requiring hand masking of components which are to be free of spray, prior to spraying. For these reasons, air spraying is not particularly suitable for modern, automated production facilities.
In airless spraying of conformal coating material, as described in the U.S. Pat. No. 4,880,663 patent, the coating material is sprayed onto a printed circuit board in a flat unatomized pattern. Relative movement is effected between the nozzle and the circuit board in a direction transverse to the plane of the flat pattern discharged from the nozzle. The supply of coating material to the nozzle is intermittently interrupted so as to prevent a deposit of liquid coating on regions of the printed circuit board and/or circuit components which are left to be uncoated. Airless spray coating of conformal coatings on printed circuit boards by this technique, however, was typically accomplished with solvent based, lower viscosity (under about 60 centipoise) coating materials. When these materials were applied at a pressure of about 20 to 30 pounds per square inch (psi), a high quality coating was achieved. A high quality coating is typically characterized by a relatively uniform thickness and an absence of overspray and liquid splatter.
Recently, because of environmental concerns, there has been a trend to reduce or eliminate the use of solvents, previously used to lower the viscosity of the otherwise high viscosity, i.e., above about 60 centipoise (cps), coating materials. To apply a high viscosity conformal material with the described airless technique, a higher application pressure (generally above about 100 psi) is required to drive the viscous material through the nozzle and cause it to form the desired film pattern. These higher pressures, however, cause a higher flow rate and result in a thicker coating (typically above about 7 mils) on the circuit board. Because of the high cost for conformal material and industry standards requiring a coating thickness of preferably about 1 to 3 mils for urethanes, acrylics and epoxies and about 2-8 mils for silicones, simply applying these low solvent coatings at a higher pressure is unsatisfactory.